The present invention relates to novel photochromic articles that activate or become darker behind an ultraviolet radiation blocking transparency, e.g., an automotive windshield. More particularly, this invention relates to photochromic articles that demonstrate activated coloration or darkness when tested under conditions simulating an eyeglass wearer behind an automotive windshield and at least 70 percent transmittance in the unactivated state.
Photochromism is a phenomenon involving a light induced reversible change in color. An article containing such a material that becomes colored upon exposure to light radiation containing ultraviolet rays will revert to the original color when the influence of the ultraviolet radiation is discontinued. Sources of light radiation that contain ultraviolet rays include, for example, sunlight and the light of a mercury lamp. Discontinuation of the ultraviolet radiation can be achieved for example by storing the photochromic material or article in the dark or by removing the source of ultraviolet radiation, e.g., by means of filtering.
Typical photochromic articles do not activate or demonstrate negligible activation behind automotive windshields due to the limited ultraviolet radiation available for activation. A photochromic article that has been disclosed to function behind such ultraviolet radiation blocking transparencies includes photochromic compounds that activate by means of light in the visible spectrum. Such an article requires the presence of ultraviolet light absorbers to prevent the article from becoming too dark when not behind the windshield. The unactivated state of such an article is colored resulting in a percent transmission less than 70 percent.
The present invention provides a photochromic article that activates behind ultraviolet radiation blocking transparencies and a method for preparing such articles using photochromic materials that have been adapted to change from an unactivated form to an activated form by exposure to activating radiation substantially in the range of from 380 to 410 nanometers (nm), when such activating radiation is measured over a range from 380 to 700 nm.